So, what's interesting is that static typing buys you somewhat minimal gains in performance. This is why dart 1 shipped with what they called "optional typing".

The thing Dart "fixed" in terms of performance is it forced consistent typing. It removed the ability to add/remove/change fields/methods on an object at runtime. The Achilles' heel of javascript (at least, when I got hype on dart years ago) is how stupidly easy it is to change the memory shape of any object. That means the VM can't generally lay out memory for an object in a compact form. Further, the VM has to do a bunch of checks before it can go down the optimized path (in case assumptions are invalidated).

For a consistent type system, the only check needed is "is this object shaped like I think it should be?" and then you go from there.

I think the most disappointing part of Dart is that all that effort was spent creating a new language when what the browser needed (and still needs) is a new bytecode. I'd love to see WASM reach the point of a universal bytecode but fear that it painted itself into a corner by first targeting memory managed languages.